Keyed multifrequency negative resistance apparatus



A. c.. scHRoEDER 2,389,004

Nov. 13, 1945.

KEYED MULTIFREQUENGY NEGATIVE RESISTANCE APPARATUS Filed June 26, 1942 E ig. E.

Suventor LmED AfcHRoEDER Parenteel Nev. 13,Y 194s KEYED MULTIFREQUENCYNEGATIVE RESISTANCE APPARATUS Alfred C. Schroeder, Feasterville, Fa., assignor to ARadio Corporation of America, a corporation of Delaware Application J une 26,- 1942, Serial No. 448,563

(ol. 25o-17) 8 Claims.

This invention relates generally to oscillation circuits and particularly to an improved method of and means for generating keying oscillations derived from a negative resistance device coupled to a multi-resonant circuit.

While various multi-frequency oscillation circuits are known in the art, the instant invention comprises a novel and simple method of keying such circuits. It comprises a negative resistance device coupled to a multiesonant circuit, wherein the normally oscillat Y circuit is blocked by keying impulses, and oscillations at the several resonant frequencies are resumed at the instant the keying signal ceasesr One characteristic of the apparatus is that oscillations at each frequency are always resumed in the same phase as previous oscillations at the same frequency. Also the separate frequencies which are generated may be keyed separately by pulses of similar frequencies, since the Q of the separate resonant circuits will prevent response to keying impulses of higher frequency than circuit resonance.

Such a circuit has considerable utility in testing equipment, particularly with respect to the application of such testing equipment to television and similar work. 'I'he circuit to be described, which is merely illustrative of the invention, includes provision for producing selectively oscillations at one or more resonant frequencies, in which such oscillations may be of differentl amplitudes. Provision is also included for keying selectively oscillations produced by any desired portion of the multi-resonant circuit, while not aecting oscillations produced by the remaining components of the multi-resonant circuit.

Among the objects of the invention are to provide a new and improved method of and means for keying selectively the oscillations of a multifrequency generator. Another object is to provide selective keying of the oscillations of a multifrequency generator, inwhich the oscillations at the several resonant frequencies may be of different amplitude. Still another object is to provide an improved method of ,and means for keying selectively some of the frequencies of a multifrequency generator, while the keying impulses are ineffective with' respect to the remaining resonant frequencies of the generator. Another object is to provide an improved method of and means for keying a multi-frequency generator in which th'e sensitivity of the circuit to predetermined keying impulses may be readily adjusted. A further object is to provide an improved method of and means for generating simultaneously a plurality of currents of different frequency by means including a negative resistance device coupled to a multi-resonant circuit.

The invention will be described by reference to the accompanying drawing, of which Fig. 1 is a schematic circuit diagram of the general form connected to the negative resistance device I.

of the invention, and Fig. 2 is a schematic circuit diagram of one preferredembodiment thereof. Similar reference numeralsare applied to similar elements throughout the drawing.

Referring to Fig. 1, a negative resistance device I is keyed by signals from a sourcev 2. A first resonant circuit-comprising a first inductance 3, a first capacitor l and a first resistor 5, connected in parallel-is connected serially to a second resonant circuit. The second resonant circuit includes a second inductance 6, a second capacitor 1 and a second resistor 8. These elements 8, 1, 3 are connected in parallel; and the circuit is also connected serially to a third resonant circuit. The third resonant circuit includesa third inductance 9, a third capacitor I0 and a third resistor I I. Th'ese elements 9, I0, II are con-` nected in parallel. The remaining terminals X and Y of the first and third resonant circuits are It should be understood that as many resonant circuits as desired may be serially, or otherwise, connected together and coupled to the negative resistance device at the terminals A, A to provide a multi-frequency generator having the/ characteristlcs hereinafter set forth.

Referring to Fig. 2 a negative resistance device,

such as a multi-grid thermiunic tube 2|, lincludes in its cathode circuit acathode resistor 22 which is connected to ground. A cathode by-pass capacitor 23 is connected in parallel with th'e cathode resistor 22. A control electrode resistor 24 is connected between the control electrode of the tube 2| and an adjustable contact on ,the cathode resistor 22. Keying signals I2, which may, for example, comprise low amplitude, high frequency negative pulses I3, and relatively high amplitude, low frequency pulses I4, are applied to the control electrode circuit through a grid coupling capacitor 25.

The negative terminal of a source of anode potential is connected to ground, and to oneterminal of a fourth resistor 26 forming a portion of voltage divider which includes the serially connected fourth and fifth resistors 26 and 2l. 'I'he positive terminal of the anode voltage source is connected to the remaining terminal of the fifth resistor 21. A by-'pass capacitor 28 is connected across th'e resistor 26. The anode of the thermionic tube 2l is connected through a low resistance 29 to the ungrounded terminal of the fourth resistor 26. An adjustable contact 30 on the fifth resistor 21 is connected to one terminal on a second by-pass capacitor 3|, the other terminal of which is grounded.

The sliding contact 30 also is connected to one terminal A of a first resonant circuit, comprising the parallel connected rst and second tuning capacitors 32, 33, fourth inductance 34 and a variable resistor 35. The remaining terminal of the first resonant circuit is connected to one terminal. of a second. resonant circuit, comprising the parallel connected third tuning capacitor 36, fth inductance 31 and a variable resistor 38. 'I'he remaining terminal A' of the second resonant circuit is connected to the screen electrode of the thermionic tube 2|. The suppressor electrode of the thermionic tube 2| is coupled -to the screen-electrode by means of a .25 mfd. capacitor 39, and is connected to ground through a high resistance 40. The iirst resonant circuit which may, for example, be tuned to one megacycle, includes a first shorting switch 4|. The second resonant circuit which may, for example, be tuned to an audible frequency such as 2.5 kilocycles, includes a second shorting switch 42, An output circuit, not shown, may be coupled to the tube anode circuit through a suitable output coupling capacitor 43, or it may be coupled in any Well known manner. In general, the particular negative resistance device described resembles the "Transitron device described in the February, 1939, issue of the Proceedings of I. R, E., pages 88-94.

In operation, the Q of the resonant circuits may be varied separately by adjusting the parallel resistors 35 and 38, or if desired, either resonant circuit may .be short-circuited by ineans of the switches 4| and 42. It should be understood that the Q of the several resonant circuits must' be controlled Within predetermined limits for satisfactory multi-frequency operation The bias on the thermionic tube control electrode is adjusted, by the tap on the cathode resistor 22, to provide normal oscillation of the circuit at the resonant frequencies of the multi-resonant circuit. The amplitude of oscillation at the several frequencies will depend, within predetermined lirnits, upon the Q of the individual resonant circuits.

The keying pulses applied, through the coupling capacitor 25, to the control electrode, will increase the negative bias upon the control electrode and cause the thermionic tube 2| to become non-conducting. When the keying pulses have ceased, the control electrode negative bias is reduced to the point where the tube again conducts, and oscillations simultaneously commence at the resonant frequencies at the multi-resonant circuit. kBy suitable control of the normal bias on the control electrode, by means of the adjustable tap on the cathode resistor, the circuit may be made to respond to keying impulses of only a predetermined minimum amplitude. For example, the circuit may be made to respond to the low frequency, high amplitude negative pulse H, while being effectively unresponsive to the high frequency low amplitude negative pulses I3. Similarly, due to the different time constants of the several resonant circuits, keying pulses of high frequency, for example, will not key oscillations of substantially lower frequency. 'Therefore, the keying frequencies, as well as the keying amplitudes may be determined for selective keying of the several resonant frequencies.

It should be'understood that the circuit deized for generating continuous modulated frequencies having any desired modulation components.

I claim as my invention:

1. Multi-frequency negative resistance apparatus including a multi-resonant circuit, a negative resistance device, means connecting said device to said circuit, connections for a source of keying signals comprising potentials of predetermined polarity and period, and means for applying said keying signals to said device to selectively derive oscillations at all of the resonant frequencies of said circuit.

2. Apparatus of the type described in claim l including adjustable bias means and means connecting said bias means to said device for conditioning said apparatus to be keyed effectively only by predetermined keying signals,

3. Apparatus of the type described in claim l including means for varying selectively the ratio of reactance to resistance in the several resonant elements of said multi-resonant circuit,

4. Apparatus of the type described in claim 1 including means for adjusting separately the resonant frequencies of the several elements of said multi-resonant circuit.

5. Apparatus of the type described in claim l including means for coupling a load circuit to said negative resistance device.

6. Multi-frequency negative resistance apparatus including a multi-resonant circuit, a negative resistance device, means connecting said device to said circuit, connections for a sourceof keying signals comprising potentials of predetermined polarity and period, means for applying said keying signals to said device to selectively derive oscillations at all of the resonant frequencies of said circuit, adjustable bias means, means connecting said-bias means to said device for conditioning said apparatus to be keyed effectively only by predetermined keying signals, means for selectively adjusting the impedance of said multi-resonant circ-uit at any predetermined frequencies, means for selectively adjusting the resonant frequencies of said multi-resonant circuit, and means for coupling a load circuit to said apparatus.

7. In apparatus including a negative resistance device connected to a multi-resonant circuit, and a source of keying signals comprising potentials of predetermined polarity and period, the method comprising conditioning said negative resistance device to generate currents dependent on each of the resonances of said multi-resonant circuit, and applying said keying signals to said device to suppress said currents during said signals and to synchronously and cc-phasally start all of said current generation simultaneously with the cessation of said signals,

8. A keyed oscillator including a multi-resonant circuit, a negative resistance device, means connecting said device to said circuit, connections for a source of keying signals comprising potentials of predetermined polarity and period, and means for applying said signals tc said negative resistance device for suppressing oscillations in said circuit for the duration of said signals and for synchronously and co-phasally starting all of said oscillations simultaneously with the cessation of said signals.

ALFRED C. SCHROEDER. 

